Python main function

Main function is the entry point of any program. But python interpreter executes the source file code sequentially and doesn’t call any method if it’s not part of the code. But if it’s directly part of the code then it will be executed when the file is imported as a module. That’s why there is a special technique to define main method in python program, so that it gets executed only when the program is run directly and not executed when imported as a module. Let’s see how to define python main function in a simple program. python_main_function.py

print("Hello")

print("__name__ value: ", __name__)


def main():
    print("python main function")


if __name__ == '__main__':
    main()

• When a python program is executed, python interpreter starts executing code inside it. It also sets few implicit variable values, one of them is __name__ whose value is set as __main__.

• For python main function, we have to define a function and then use if __name__ == '__main__' condition to execute this function.

• If the python source file is imported as module, python interpreter sets the __name__ value to module name, so the if condition will return false and main method will not be executed.

• Python provides us flexibility to keep any name for main method, however it’s best practice to name it as main() method. Below code is perfectly fine, however not recommended.

  def main1():
      print("python main function")
  
  
  if __name__ == '__main__':
      main1()
  

Below image shows the output when python_main_function.py is executed as source file.

Python main function as module

Now let’s use above python source file as a module and import in another program. python_import.py

import python_main_function

print("Done")

Now when above program is executed, below output is produced.

Hello
__name__ value:  python_main_function
Done

Notice that first two lines are getting printed from python_main_function.py source file. Notice the value of __name__ is different and hence main method is not executed. Notice that python program statements are executed line by line, so it’s important to define the main() method first before the if condition to execute main method. Otherwise you will get error as NameError: name 'main' is not defined. That’s all about python main function. Reference: Python Docs